Historically, polyethylene terephthalate has been difficult to mold at molding temperatures below about 110.degree. C. because its crystallization rate is so slow and uneven that warped articles are obtained. Moreover the articles tend to stick to the mold and are difficult to remove. Recently, it has been found that polyethylene terephthalate articles of good quality can be obtained by molding at lower temperatures, e.g., 80.degree.-100.degree. C., if certain materials are added to the resin prior to molding. These additive materials increase the rate of crystallization, and molded articles having smooth glossy surfaces that are easily removable from molds, are obtained when the materials are employed. The additive materials are (1) a sodium or potassium salt of a selected hydrocarbon acid or a sodium or potassium salt of a selected organic polymer containing pendant carboxyl groups and (2) a selected low molecular weight organic compound that is an ester, ketone, sulfone, sulfoxide, nitrile or amide.
However, many of these low molecular weight organic compounds are somewhat volatile at molding temperatures and tend to vaporize and redeposit on the mold surfaces. It would be advantageous to find a low molecular weight organic compound that does not so vaporize and redeposit.